You are also told to subtract PITI (at 1% of value) to determine worst case scenario cash flow, per month.I like the idea, but the the numbers seem a bit low, particularly compared to property value in my market (Cayman Islands).

Once you do that, you need to subtract out the repair costs to come to a more realistic price.

Subtract your mortgage amount from the cash flow number I had above and that's your cash flow.

I can get the basic principle of taking your annual rents and subtracting insurance, property taxes, expenses, etc. 

Then subtract all expenses from the sale price and split the profit.  

Most houses will need at least some repair, if not a lot, so you will also need to subtract that from your number (if it will cost $20,000 to get the house into shape, then $100,000 x 0.7 = $70,000 - $5,000 - $20,000 = $45,000).  

To get a real value, you need to look at sold comps and subtract out any buyer kickback from those numbers, adjust for amenities, and adjust for condition/repairs/etc. 

Currently, AFTER mortgage,interest,insurance,taxes as well as prop.management fees, vacancy factor saving(150/month), maintenance/cap ex. saving($100/month), and utilities are all subtracted from rents, the property cash flows $250/month.If I refi and take out $50,000 for an additional property, the refinance for the duplex using all above expense numbers plus the now higher mortgage will result in only $20/month cash flow.So, is it worth it and a good idea to take out this money?

Don't connect the two.To figure out the lease payment you will be making, take your rental income as a starting point and subtract all the expenses you will be handling (paying), then the desired cash flow (if you don't make positive cash flow, why bother).  

To get the answer you need to know the comparable sales, the mortgage balance and other encumbrances attached to the property.Add - Subtract - Total = Equity  (sort of)Some basic questions:How fast do you want to get to the settlement table?